Bulk Resonator with Symmetrically Positioned Temperature Compensation Layers

ABSTRACT

A bulk acoustic wave (BAW) resonator with better performance and better manufacturability is described. A BAW resonator includes a substrate, a BAW stack disposed over the substrate, a first temperature compensation layer disposed between the substrate and the stack, and a second temperature compensation layer disposed over the stack. The BAW stack includes a piezoelectric layer disposed between a first electrode and a second electrode. A method of making a BAW resonator is also disclosed. The method includes forming a first base layer over a substrate including a layer of sacrificial material and a frame surrounding the layer of sacrificial material, forming a first temperature compensation layer over the first base layer, forming a BAW stack over the first temperature compensation layer, forming a second temperature compensation layer over the BAW stack, and removing the layer of sacrificial material to form a cavity adjacent the base layer.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/915,581, filed Oct. 15, 2019, U.S. Provisional Patent Application No. 62/915,573, filed Oct. 15, 2019, U.S. Provisional Patent Application No. 62/915,577, filed Oct. 15, 2019, and U.S. Provisional Patent Application No. 62/915,588, filed Oct. 15, 2019, each which is hereby incorporated by reference in its entirety.

This application is related to U.S. patent application Ser. No. ______, filed ______, (Attorney Docket No. 020762-5008), entitled “Bulk Acoustic Wave Resonator with Multilayer Base,” U.S. patent application Ser. No. ______, filed ______, (Attorney Docket No. 020762-5009), entitled “Composite Piezoelectric Film and Bulk Acoustic Resonator Including Same,” and U.S. patent application Ser. No. ______, filed ______, (Attorney Docket No. 020762-5011), entitled “Bulk Acoustic Resonator Structures with Improved Edge Frames,” each of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The disclosed embodiments relate generally to bulk acoustic wave resonators, and in particular, to bulk acoustic wave resonators that include symmetrically positioned temperature compensation layers and method of making thereof.

BACKGROUND

Bulk acoustic wave (BAW) resonators are widely used in RF filters in mobile devices due to their compact size and high performance. A BAW resonator typically includes a piezoelectric thin film layer between a bottom electrode and a top electrode. Piezoelectric thin film materials used for bulk acoustic wave devices include AlN, ZnO thin films for small bandwidth applications and PZT films for wide bandwidth applications. When an oscillating electrical signal is applied between the top and bottom electrodes, the piezoelectric thin film layer converts the oscillating electrical signal into bulk acoustic waves.

The resonance frequency of the BAW resonator is mainly determined by the acoustic velocity and thickness of the piezoelectric layer and the electrodes, but can be susceptible to changes in ambient temperature. For example, a typical BAW resonator can have a temperature coefficient of frequency (TCF) about −45 ppm/° C. without some form of temperature compensation mechanisms. Temperature compensation is needed if temperature-induced frequency shift causes the pass band and rejection band of a BAW filter to be out of specified tolerance.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the present disclosure can be understood in greater detail, a more particular description may be had by reference to the features of various embodiments, some of which are illustrated in the appended drawings. The appended drawings, however, merely illustrate pertinent features of the present disclosure and are therefore not to be considered limiting, for the description may admit to other effective features.

FIG. 1 is a cross-sectional diagram of a bulk acoustic wave resonators, in accordance with some embodiments.

FIGS. 2A-2G are cross-sectional diagrams illustrating formation of the bulk acoustic resonator, in accordance with some embodiments.

FIG. 3 is a flow diagram of a method of making the bulk acoustic wave resonator, in accordance with some embodiments.

FIG. 4 is a bird eye's view of the bulk acoustic wave resonator, in accordance with some embodiments.

In accordance with common practice, the various features illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. In addition, some of the drawings may not depict all of the components of a given system, method or device. Finally, like reference numerals may be used to denote like features throughout the specification and figures.

DETAILED DESCRIPTION

The various embodiments described herein include systems, methods and/or devices with structures for improved performance and manufacturability.

Numerous details are described herein in order to provide a thorough understanding of the example embodiments illustrated in the accompanying drawings. However, some embodiments may be practiced without many of the specific details, and the scope of the claims is only limited by those features and aspects specifically recited in the claims. Furthermore, well-known processes, components, and materials have not been described in exhaustive detail so as not to unnecessarily obscure pertinent aspects of the embodiments described herein.

A1. Some embodiments include a bulk acoustic resonator, comprising: a substrate; a stack over the substrate, the stack including a first electrode, a second electrode, and a piezoelectric layer between the first electrode and the second electrode; a first temperature compensation layer disposed between the substrate and the stack; and a second temperature compensation layer over the stack.

A2. In some embodiments of the bulk acoustic resonator of A1, the substrate includes a cavity and a frame around the cavity; and the stack bridges the cavity and is supported by the frame.

A3. In some embodiments of the bulk acoustic resonator of any of A1 and A2, at least one of the first temperature compensation layer and the second temperature compensation layer includes silicon dioxide (SiO₂).

A4. In some embodiments of the bulk acoustic resonator of any of A1 and A2, at least one of the first temperature compensation layer and the second temperature compensation layer includes tellurium dioxide (TeO₂).

A5. In some embodiments of the bulk acoustic resonator of any of A1-A4, the first temperature compensation layer and the second temperature compensation layer include a same material.

A6. In some embodiments of the bulk acoustic resonator of any of A1-A5, the piezoelectric layer has a positive temperature coefficient of frequency, and each of the first temperature compensation layer and the second temperature compensation layer has a negative temperature coefficient of frequency.

A7. In some embodiments of the bulk acoustic resonator of any of A1-A6, the piezoelectric layer is configured to resonate at frequencies within a predetermined frequency band, and the first temperature compensation layer and the second temperature compensation layer are disposed symmetrically with respect to an active region of the piezoelectric layer to reduce resonance at frequencies outside of the predetermined frequency band.

A8. In some embodiments of the bulk acoustic resonator of any of A1-A7, the first temperature compensation layer and the second temperature compensation layer have a same thickness.

A9. In some embodiments, the bulk acoustic resonator of any of A1-A8 further comprises a first base layer disposed between the substrate and the first temperature compensation layer.

A10. In some embodiments of the bulk acoustic resonator of A9, the first base layer includes aluminum nitride (AlN).

A11. In some embodiments, the bulk acoustic resonator of any of A1-A10 further comprises a second base layer disposed between the first temperature compensation layer and the stack.

A12. In some embodiments of the bulk acoustic resonator of A11, the second base layer includes aluminum nitride (AlN).

A13. In some embodiments, the bulk acoustic resonator of any of A1-A12 further comprises a passivation layer disposed over the second temperature compensation layer.

A14. In some embodiments of the bulk acoustic resonator of A13, the passivation layer includes aluminum nitride (AlN).

A15. In some embodiments of the bulk acoustic resonator of any of A13-A14, the passivation layer has a thickness equal to a thickness of the first base layer.

A16. In some embodiments of the bulk acoustic resonator of any of A13-A14, the passivation layer has a thickness that is equal to a thickness of the first base layer plus a thickness of the second base layer.

A17. Some embodiments provide a method of making bulk acoustic resonator, comprising: forming a first temperature compensation layer over a substrate, the substrate including a layer of sacrificial material and a frame surrounding the layer of sacrificial material, forming a stack over the first temperature compensation layer, the stack including a first electrode, a second electrode, and a piezoelectric layer between the first electrode and the second electrode; forming a second temperature compensation layer over the stack; and removing the layer of sacrificial material to form a cavity adjacent the first base layer.

A18. In some embodiments, the method of A17 further comprises: forming a first base layer over the substrate before forming the first temperature compensation layer, wherein the first base layer is formed over the layer of sacrificial material and the frame and wherein the first temperature compensation layer is formed over the first base layer; and/or forming a second base layer over the first temperature compensation layer before forming the stack, wherein the stack is formed over the second base layer.

A19. In some embodiments, the method of any of A17 and A18 further comprises forming a passivation layer over the second temperature compensation layer before removing the layer of sacrificial material.

A20. In some embodiments of the method of any of A17-A19, the piezoelectric layer has a positive temperature coefficient of frequency, and each of the first temperature compensation layer and the second temperature compensation layer has a negative temperature coefficient of frequency.

FIG. 1 is a cross-sectional diagram of a bulk acoustic wave (BAW) resonator 100, in accordance with some embodiments. BAW resonator 100 includes a substrate 102, a stack 110 disposed over the substrate 102, a first temperature compensation layer 120 disposed between the substrate 102 and the stack 110, and a second temperature compensation layer 122 disposed over the stack 110. The stack 110 includes a first electrode 112, a second electrode 114 over the first electrode 112, and a piezoelectric layer 116 between the first electrode 112 and the second electrode 114.

In some embodiments, the piezoelectric layer 116 may be a single layer of piezoelectric material or a multilayer piezoelectric composite of two or more piezoelectric materials. For example, the piezoelectric layer 116 can be any of the composite piezoelectric film described in co-pending U.S. patent application entitled “Composite Piezoelectric Film and Bulk Acoustic Resonator Including Same,” filed on even date herewith, which is incorporated herein by reference. In some embodiments, the piezoelectric layer 116 includes aluminum nitride (AlN) and/or scandium aluminum nitride (Sc_(x)Al_(1-x)N, where x is between 1% and 45%).

In some embodiments, the BAW resonator 100 further includes a first base layer 130 disposed between the substrate 102 and the first temperature compensation layer 120. In some embodiments, the first base layer 130 includes aluminum nitride (AlN).

In some embodiments, the BAW resonator further includes a second base layer 132 disposed between the first temperature compensation layer 120 and the stack 110. In some embodiments, the second base layer 132 includes aluminum nitride (AlN).

In some embodiments, either or both of the first base layer 130 and the second base layer 132 can be a multilayer base, such as those described in co-pending U.S. patent application, entitled “Bulk Acoustic Wave Resonator with Multilayer Base,” filed on even date herewith, which is hereby incorporated by reference in its entirety.

In some embodiments, the BAW resonator 100 further includes a passivation layer 134 disposed over the second temperature compensation layer 122. In some embodiments, the passivation layer 134 includes aluminum nitride (AlN).

In some embodiments, the passivation layer 134 has a thickness D1 equal to a thickness D2 of the first base layer 130 (e.g., the difference between D1 and D2 is less than 10% or 5% of either D1 or D2 or an average of both).

In some embodiments, the thickness D1 of the passivation layer 134 is equal to a thickness D3 of the second base layer 132 (e.g., the difference between D1 and D3 is less than 10% or 5% of either D1 or D3 or an average of both).

In some embodiments, the passivation layer 134 has a thickness D1 that is equal to a thickness D2 of the first base layer 130 plus a thickness D3 of a second base layer 132 (e.g., the difference between [D1+D2] and D3 is less than 10% or 5% of either [D1+D2] or D3 or an average of both [D1+D2] and the D3).

In some embodiments, the substrate 102 includes a cavity 104 and a frame 106 around the cavity. The stack 110 bridges the cavity 104 and is supported by the frame 106. In some embodiments, the substrate 102 includes silicon and\or polysilicon (e.g., frame 106 includes polysilicon).

In some embodiments, at least one of the first temperature compensation layer 120 and the second temperature compensation layer 122 includes silicon dioxide (SiO₂).

In some embodiments, at least one of the first temperature compensation layer 120 and the second temperature compensation layer 122 includes tellurium dioxide (TeO₂).

In some embodiments, the first temperature compensation layer 120 and the second temperature compensation layer 122 include a same material.

In some embodiments, the piezoelectric layer 116 has a positive temperature coefficient of frequency, and each of the first temperature compensation layer 120 and the second temperature compensation layer 122 has a negative temperature coefficient of frequency.

In some embodiments, the first temperature compensation layer 120 and the second temperature compensation layer 122 have a same thickness (e.g., the difference between a thickness D4 of the first temperature compensation layer 120 and a thickness D5 of the second temperature compensation layer 122 is less than 10% or 5% of either D4 or D5 or an average of D4 and D5).

Referring to FIG. 1, in some embodiments, the piezoelectric layer 116 is configured to resonate at frequencies within a predetermined frequency band, and the first temperature compensation layer 120 and the second temperature compensation layer 122 are disposed symmetrically with respect to an active region of the piezoelectric layer 116, so as to reduce resonance at frequencies outside of the predetermined frequency band. For example, the first temperature compensation layer 120, and the second temperature compensation layer 122 are symmetric with respect to a virtual middle plane 150 of the stack 110 (e.g., a horizontal virtual plane that divides the piezoelectric layer 116 into two equal parts). For example, a first distance d1 between a bottom surface 120-b of the first temperature compensation layer 120 and the virtual middle plane 150 is the same or about the same as a second distance d2 between a top surface 122-t of the second temperature compensation layer 122 and the virtual middle plane 150 (e.g., a difference between d1 and d2 is less than 10% or less than 5% of d1 or 2 or an average of d1 and d2).

The bulk acoustic wave resonator 100 can be fabricated using a process illustrated in FIGS. 2A-2G and 3, according to some embodiments. FIGS. 2A-2G illustrate cross-sectional views of a bulk acoustic resonator during formation of the bulk acoustic resonator 100. FIG. 3 illustrates a flowchart representation of a method 300 for forming a BAW resonator (e.g., resonator 100) in accordance with some embodiments.

As shown in FIG. 2A, a substrate 102 is prepared. The substrate 102 includes a layer of sacrificial material 201 and a frame 106 surrounding the layer of sacrificial material 201. The first base layer 130 is formed over the layer of sacrificial material 201 and the frame 106. The layer of sacrificial material 201 (e.g., silicon dioxide) can be formed (e.g., by chemical vapor deposition and subsequent patterning) on a bulk substrate 203 (e.g., silicon, glass, ceramic, gallium arsenide and/or silicon carbide). The sacrificial material is patterned (e.g., using a mask and chemical etching) such that sacrificial material 201 occupies an area on the bulk substrate 203 that corresponds to the ultimate location of cavity 104. The cavity frame 106 can be formed (e.g., by e-beam evaporation) around sacrificial material 201. The cavity frame is then patterned (e.g., using a mask during the e-beam evaporation) such that cavity frame 106 forms a perimeter around sacrificial material 201. A planarizing material 205 (e.g. polysilicon) can be formed on the bulk substrate 203, cavity frame 106, and sacrificial material 201 (e.g., by chemical vapor deposition), and subsequently planarized (e.g., by chemical mechanical polishing) to form a level upper surface 207 for the substrate 102, which includes planarizing material 205, cavity frame 106, and sacrificial material 802 on the bulk substrate 203.

As shown in FIGS. 2B and 3, the method 300 includes (step 310) forming (e.g., by physical vapor deposition or epitaxial growth) a first base layer 130 over the substrate 102. In some embodiments, the first base layer 130 includes aluminum nitride (AlN). In some embodiments, forming the first base layer 130 includes forming a multilayer base, described in the co-pending U.S. patent application, entitled “Bulk Acoustic Wave Resonator with Multilayer Base,” filed on even date herewith.

In some embodiments, as shown in FIGS. 2C and 3, method 300 further includes (step 320) forming (e.g., by chemical vapor deposition) a first temperature compensation layer 120 over the first base layer 130. In some embodiments, the first temperature compensation layer 120 includes silicon dioxide (SiO₂).

As shown in FIGS. 2D and 3, method 300 further includes (optional step 322) forming a second base layer 132 after forming the first temperature compensation layer 120 and before forming the stack 110, and the stack 110 is formed over the second base layer 132. In some embodiments, the second base layer 130 includes aluminum nitride (AlN). In some embodiments, forming the second base layer 130 includes forming a multilayer base, described in the co-pending U.S. patent application, entitled “Bulk Acoustic Wave Resonator with Multilayer Base,” filed on even date herewith.

As shown in FIGS. 2E and 3, method 300 also includes (step 330) forming a stack 110 over the first temperature compensation layer 120 or the second base layer 132. Forming the stack 110 includes forming (e.g., by physical vapor deposition) a bottom electrode layer 112 (e.g., molybdenum, aluminum, and/or tungsten) over the first temperature compensation layer 120, forming (e.g., by physical vapor deposition or epitaxial growth) a piezoelectric layer 116 (e.g., aluminum nitride (AlN) and/or scandium aluminum nitride (Sc_(x)Al_(1-x)N, where x is between 1% and 45%)) over the bottom electrode layer 112, and forming (e.g., by physical vapor deposition) a top electrode layer 114 (e.g., molybdenum, aluminum, and/or tungsten) over the piezoelectric layer 116. In some embodiments, the bottom electrode layer 112 is patterned (e.g., using a mask during the physical vapor deposition) such that bottom electrode layer 112 occupies a region indicated by bottom electrode layer 112 in FIG. 2E. In some embodiments, the top electrode layer 114 is patterned (e.g., using a mask during the physical vapor deposition) such that top electrode layer 144 occupies the region indicated by top electrode layer 144 in FIG. 2E. In some embodiments, the piezoelectric layer 116 includes a composite piezoelectric film and forming the piezoelectric layer 116 includes any of the processes for forming a composite piezoelectric film, as described in the co-pending U.S. patent application entitled “Composite Piezoelectric Film and Bulk Acoustic Resonator Including Same,” filed on even date herewith.

As shown in FIGS. 2F and 3, method 300 also includes (step 340) forming (e.g., by chemical vapor deposition) a second temperature compensation layer 122 over the stack 110. In some embodiments, the second temperature compensation layer 122 includes silicon dioxide (SiO₂). In some embodiments, the piezoelectric layer 116 has a positive temperature coefficient of frequency, and each of the first temperature compensation layer 120 and the second temperature compensation layer 124 has a negative temperature coefficient of frequency.

In some embodiments, as shown in FIGS. 2G and 3, method 300 further includes (optional step 342) forming a passivation layer 134 over the second temperature compensation layer 122.

As shown in FIGS. 2G and 3, method 300 also includes (step 350) removing the layer of sacrificial material 201 to form a cavity 104 adjacent the first base layer 130. In some embodiments, cavity 104 is formed by removing the sacrificial material 201 (e.g., by vapor HF etching) from first base layer 130. Vapor HF etching advantageously reduces the etch time (e.g., compared with liquid HF) and provides a clean surface of the bottom electrode. In some embodiments, the cavity 104 has a depth and shape that corresponds to the opening of cavity frame 106. In this way, the formation of cavity frame 106 allows formation of a cavity with a predetermined depth and shape.

While the above sequences of operations, and the resulting bulk acoustic resonators, are new, the techniques needed to perform each of the individual steps or operations of these processes are well understood in the art, and therefore the individual processing steps or operations are not described in detail. The dashed lines in method 300 illustrate optional or alternative operations.

FIG. 4 is a bird's-eye view of resonator 100 before removal of the sacrificial layer, in accordance with some embodiments. Region 410 corresponds to a portion of the substrate 102 that includes the sacrificial material. The sacrificial material in region 410 is covered by the first base layer 130. A plurality of injection regions 412-1 through 412-n correspond to portions of the substrate 102 that allow access to the sacrificial material. In some embodiments, as shown in FIG. 4, the plurality of regions 412-1 through 412-n correspond to portions of the substrate 102 that include sacrificial material that is not covered by the stack 110. In some embodiments, removing the sacrificial material under the stack to form the cavity 104 may include submerging the substrate 102 with the layers formed thereon in a chemical etchant (e.g., hydrofluoric acid or HF liquid or vapor). Using vapor HF etching to remove the sacrificial SiO₂ layer can reduce the etch time and provides a clean surface of the bottom electrode, as described in U.S. patent application Ser. No. 16/455,627 entitled “Support Structure for Bulk Acoustic Resonator,” filed Jun. 27, 2019, which incorporated herein by reference in its entirety. The etchant can access the sacrificial material under the stack via regions 412 outside of the stack, while the temperature compensation layers 120 and 130 are protected from the etchants by the first base layer 120 and the passivation layer 134.

It will be understood that, although the terms “first,” “second,” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the claims. As used in the description of the embodiments and the appended claims, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

As used herein, the term “if” may be construed to mean “when” or “upon” or “in response to determining” or “in accordance with a determination” or “in response to detecting,” that a stated condition precedent is true, depending on the context. Similarly, the phrase “if it is determined [that a stated condition precedent is true]” or “if [a stated condition precedent is true]” or “when [a stated condition precedent is true]” may be construed to mean “upon determining” or “in response to determining” or “in accordance with a determination” or “upon detecting” or “in response to detecting” that the stated condition precedent is true, depending on the context.

The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the claims to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain principles of operation and practical applications, to thereby enable others skilled in the art. 

What is claimed is:
 1. A bulk acoustic resonator, comprising: a substrate; a stack over the substrate, the stack including a first electrode, a second electrode, and a piezoelectric layer between the first electrode and the second electrode; a first temperature compensation layer disposed between the substrate and the stack; and a second temperature compensation layer over the stack.
 2. The bulk acoustic resonator of claim 1, wherein: the substrate includes a cavity and a frame around the cavity; and the stack bridges the cavity and is supported by the frame.
 3. The bulk acoustic resonator of claim 1, wherein: at least one of the first temperature compensation layer and the second temperature compensation layer includes silicon dioxide (SiO₂).
 4. The bulk acoustic resonator of claim 1, wherein: at least one of the first temperature compensation layer and the second temperature compensation layer includes tellurium dioxide (TeO₂).
 5. The bulk acoustic resonator of claim 1, wherein: the first temperature compensation layer and the second temperature compensation layer include a same material.
 6. The bulk acoustic resonator of claim 1, wherein: the piezoelectric layer has a positive temperature coefficient of frequency; and each of the first temperature compensation layer and the second temperature compensation layer has a negative temperature coefficient of frequency.
 7. The bulk acoustic resonator of claim 1, wherein: the piezoelectric layer is configured to resonate at frequencies within a predetermined frequency band; and the first temperature compensation layer and the second temperature compensation layer are disposed symmetrically with respect to an active region of the piezoelectric layer to reduce resonance at frequencies outside of the predetermined frequency band.
 8. The bulk acoustic resonator of claim 1, wherein: the first temperature compensation layer and the second temperature compensation layer have a same thickness.
 9. The bulk acoustic resonator of claim 1, further comprising: a first base layer disposed between the substrate and the first temperature compensation layer.
 10. The bulk acoustic resonator of claim 9, further comprising a passivation layer disposed over the second temperature compensation layer, wherein the passivation layer has a thickness equal to a thickness of the first base layer.
 11. The bulk acoustic resonator of claim 9, wherein the first base layer includes aluminum nitride (AlN).
 12. The bulk acoustic resonator of claim 1, further comprising: a second base layer disposed between the first temperature compensation layer and the stack.
 13. The bulk acoustic resonator of claim 12, wherein the second base layer includes aluminum nitride (AlN).
 14. The bulk acoustic resonator of claim 1, further comprising: a passivation layer disposed over the second temperature compensation layer.
 15. The bulk acoustic resonator of claim 13, wherein the passivation layer includes aluminum nitride (AlN).
 16. The bulk acoustic resonator of claim 1, further comprising: a first base layer disposed between the substrate and the first temperature compensation layer; a second base layer disposed between the first temperature compensation layer and the stack; and a passivation layer disposed over the second temperature compensation layer; wherein the passivation layer has a thickness that is equal to a thickness of the first base layer plus a thickness of the second base layer.
 17. A method of making bulk acoustic resonator, comprising: forming a first temperature compensation layer over a substrate, the substrate including a layer of sacrificial material and a frame surrounding the layer of sacrificial material; forming a stack over the first temperature compensation layer, the stack including a first electrode, a second electrode, and a piezoelectric layer between the first electrode and the second electrode; forming a second temperature compensation layer over the stack; and removing the layer of sacrificial material to form a cavity adjacent the first base layer.
 18. The method of claim 17, further comprising: forming a first base layer over the substrate before forming the first temperature compensation layer, wherein the first base layer is formed over the layer of sacrificial material and the frame, and wherein the first temperature compensation layer is formed over the first base layer; and/or forming a second base layer over the first temperature compensation layer before forming the stack, wherein the stack is formed over the second base layer.
 19. The method of claim 18, further comprising: forming a passivation layer over the second temperature compensation layer before removing the layer of sacrificial material, the passivation layer having a thickness equal to a thickness of the first base layer or to a thickness of the first base layer plus a thickness of the second base layer.
 20. The method of claim 17, wherein: the piezoelectric layer has a positive temperature coefficient of frequency; and each of the first temperature compensation layer and the second temperature compensation layer has a negative temperature coefficient of frequency. 